(a) Field of the Invention
This invention relates a turret (rotary lens exchanging device) and more particularly to a device whereby a turret board holding a plurality of lenses having different focal distances (magnifications) is stopped by a signal detected by a detecting element so that any desired lens can be positioned accurately on the light path.
(b) Description of the Prior Art
This kind of turret, for example, by Japanese Patent Laid Open No. 36614/1981 is known. It shall be explained with reference to FIG. 1. The reference numerals 1a, 1b and 1c denote lenses respectively different in the focal distance (magnification). The reference numeral 2 denotes a turret board rotatably arranged around a center axis and fitted with the lenses 1a, 1b and 1c, 3 denotes a slit plate formed separately from the turret board 2, rotatably arranged coaxially with the turret board 2 and having a tooth part 3a formed on the outer periphery and meshed with a pinion 5 of a driving moter 4, 6 denotes an interlocking pin secured to the turret board and engaged with an incision 3b formed in the slit plate 3 so as to be relatively rotatable by some range, 7 denotes a photosensor which detects slits 3c formed in response to the respective lenses 1a, 1b and 1c in the flange part of the slit plate 3 and is arranged so as to be able to detect the respective slits just before the lenses corresponding respectively to the slits 3c reach predetermined positions and 8 denotes a locking member which can engage with notches 2a formed on the outer periphery of the turret board 2 in response to the respective lenses 1a, 1b and 1c, is biased clockwise by a spring 9 and is so arranged that, when the turret board 2 is stopped in a position of engaging with the notch 2a, the lens corresponding to the notch 2a will be accurately positioned. Therefore, when the driving motor 4 is driven by a signal for commanding the conversion of the lens, the slit plate 3 will be rotated and this rotation will be transmitted to the turret board 2 through the interlocking pin 6. When the photosenser 7 detects a predetermined slit 3c, the driving motor 4 will be stopped and the slit plate 3 will stop at the same time but the turret board 2 formed so as to be able to relatively rotate in some range will continue to rotate due to the inertia within the range. When the locking member 8 engages with the notch 2a, the turret board 2 will stop accurately in a predetermined position.
However, in the above mentioned device, just before the lenses corresponding the respective slits 3c reach predetermined positions, the photosenser 7 will detect the respective slits 3c to stop the moter 4 and, by the rotating inertia of the turret board 2 and the biasing force of the locking member 8 based on the spring 9, the turret board 2 will be positioned. Therefore, the locking position of the locking member 8, the positions of the notches 2a of the turret board 2, the positions of the slits 3c of the slit plate 3 and the detecting position of the photosenser 7 must be respectively accurately determined and the assembling has been difficult. Further, as the photosenser 7 generally has an expansion of the emitted light bundle to some extent and in case the slit plate 3 rotates slowly, the light bundle detected by the photointerrupter at the time of detection will gradually increase, therefore, together with the fluctuation of the reflection factor of the light, the detecting precision will fluctuate and particularly, in case a reflective photointerrupter is used for the detecting element, the detected light will become a diffused light and has not been adapted to this kind of structure.